Prepreg composite materials have been widely used in various high-performance structures, such as aircraft and automobile components, and sport equipment (e.g., fishing rods, golf club shafts, badminton rackets, tennis rackets, etc.). A prepreg is a fiber reinforcement that is pre-impregnated with a matrix resin, typically a thermoset resin. The fibers reinforce the matrix resin, bearing the majority of the load supported by the prepreg material, while the resin matrix bears a minority portion of the load supported by the prepreg material and also transfers load from broken fibers to intact fibers. In this manner, the prepreg material can support greater loads than either the matrix resin or fibers can support alone. Furthermore, by tailoring the reinforcing fibers in a particular geometry or orientation, a composite material can be efficiently designed to minimize weight and volume while maximizing strength.
Prepregs may be manufactured by impregnating a web of continuous fibers or a fabric with a matrix resin, creating a pliable and tacky sheet of material. During impregnation, the reinforcing fibers are impregnated with the matrix resin in a controlled fashion. The precise specification of the fibers, their orientation and the formulation of the resin matrix can be specified to achieve the optimum performance for the intended use of the prepregs. The mass of fibers per square meter can also be specified according to requirements.
The term “impregnate” refers to the introduction of a matrix resin to reinforcement fibers so as to partially or fully encapsulate the fibers with the resin. The impregnation process controls the amount of resin inside the fiber bed and at the surface of the fiber bed. Furthermore, the resin impregnation level impacts the methods used to assemble the finished composite part and the part's quality. The matrix resin for making prepregs may take the form of resin films or liquids. Typically, impregnation is facilitated by the application heat and/or pressure. The resulting prepregs produced from the prepreg fabrication process is in an uncured or curable state (i.e., not hardened) and may be frozen in order to inhibit the polymerization of the resin. For manufacturing composite parts from prepregs, the cold prepregs are thawed to room temperature, cut to size, and assembled on a molding tool through various methods, such as hand layup, Automated Tape Layup (ATL), and Advanced Fiber Placement (AFP). The prepreg material for each assembly method requires different levels of impregnation and different levels of tack. Level of “tack” refers to how well prepregs stick to one another and to a tool surface. For example, for hand layup, there is less need for high level of impregnation and greater need for tack while with AFP the fiber bed requires much higher levels of impregnation. Once in place, the prepregs are consolidated and cured under pressure to achieve the required fiber volume fraction with minimal voids.
Currently, many conventional methods for impregnating continuous fiber material involve the use of static pressure-applying mechanism. Roller nips, for example, have been used to supply pressure from a fixed position in space while a continuous web moves through the static nips. These conventional processes are generally limited to a web speed of 1 to 4 m/min for high impregnation of thick resin films and fiber materials. They are also limited in their operating temperatures as higher temperatures tend to cause problems with premature curing of the resin or swelling in the case of thermoplastic resin. Essentially, what dominates the prepreg world are the fundamental physical limitations outlined in Darcy's law: the rate of fluid flow is a function of the pressure supplied, the thickness of the body, the permeability of the body of interest and the viscosity of the fluid. In the case of carbon fiber webs, the body has a dynamic permeability and the fluid has a dynamic viscosity, i.e. viscosity which changes with shear rate and temperature. This law cannot be over-ridden. Different fiber materials, different resins, different pressures and web speeds all change the shape and movement of the function but do not change the law. So a static nip or a belt under certain pressures and temperatures will always limit the production speed of the material. If the web is moving too fast, a static nip cannot press enough resin into the fiber web. If the temperature applied to the resin is too hot, the material will distort and will be ruined, and if too cold, there is insufficient force to press the resin into the fiber web.
In light of the issues discussed above, there remains a need for an improved resin impregnation technique that can increase prepreg production rate without sacrificing the control of impregnation level.